Add Code of Conduct (#396)

* Add link to Code of Conduct

We are about to add a `CODE_OF_CONDUCT.md` and it makes sense to surface it here.

* Add `CODE_OF_CONDUCT.md`

In the past there was not info and support to add the Code of Conduct Facebook Open Source projects. We can fix it. :)

**why make this change?:**
Facebook Open Source provides a Code of Conduct statement for all
projects to follow, to promote a welcoming and safe open source community.

Exposing the COC via a separate markdown file is a standard being
promoted by Github via the Community Profile in order to meet their Open
Source Guide's recommended community standards.

As you can see, adding this file will improve [the GraphQL community profile](https://github.com/facebook/graphql/community)
checklist and increase the visibility of our COC.

**test plan:**
Viewing it on my branch -
(Flarnie will insert screenshots)

**issue:**
internal task t23481323

* Improve wording of link to Code of Conduct

I think this is a bit more readable, since this `CONTRIBUTING` doc has a different flow from the usual boilerplate.

* further improvements and fix typo

Author: flarnie

CODE_OF_CONDUCT.md

@@ -0,0 +1,3 @@
+# Code of Conduct
+
+Facebook has adopted a Code of Conduct that we expect project participants to adhere to. Please [read the full text](https://code.facebook.com/codeofconduct) so that you can understand what actions will and will not be tolerated.

CONTRIBUTING.md

@@ -14,7 +14,8 @@ status and progress of proposals.
 Any changes to the specification that affect the behavior of **GraphQL server
 or client implementations** should go through the stages below no matter how
 small they are. All other changes should be labeled as “editorial” and may
-be merged right away.
+be merged right away.  Also, please read the [Code of Conduct](CODE_OF_CONDUCT.md)
+to know what is expected when making contributions to this project.
 
 ## Stage -2: proposed change (optional)